(1) Field of the Invention
The present invention relates to a field-effect transistor, and an integrated circuit device and a switching circuit using the same, and more particularly relates to a field-effect transistor used for radio frequency communications equipment or a switching circuit.
(2) Description of Related Art
In radio frequency communications equipment typified by cellular phones, there have been widely used Modulation Doped Field Effect Transistors (MODFET) using an electron supply layer doped with n-type impurities on an undoped channel layer.
Conventionally, indium gallium arsenide (InGaAs) with high electron mobility has been used for channel layers of MODFETs, and aluminum gallium arsenide (AlGaAs) having a larger band gap than InGaAs for electron supply layers.
However, the use of AlGaAs with a high interface state density makes it difficult to increase the current density of a MODFET, due to the trapping of electrons in an interface state. More specifically, an interface state is formed on the surface of an electron supply layer and serves as a trap for trapping electrons and holding them on the surface of the electron supply layer. The negative electric charge of the electrons trapped in this trap narrows a depletion layer in a channel region. A so-called frequency dispersion of drain current is caused in which such a depletion-layer-narrowing phenomenon significantly decreases the maximum current density in the application of an RF signal to a gate as compared with that in the application of a direct current thereto.
To cope with such a problem, indium gallium phosphide (InGaP), instead of AlGaAs, is used as a material constituting an electron supply layer, thereby decreasing traps produced in the electron supply layer. This can provide increased current density of a field-effect transistor (for example, see Japanese Laid-Open Patent Application Publication No. 63-228763).
FIG. 7 shows the cross sectional structure of a known MODFET using InGaP for an electron supply layer. As shown in FIG. 7, a buffer layer 102 made of undoped GaAs, an n-type-impurity-doped barrier layer 103 made of AlGaAs, a channel layer 104 made of undoped In0.2Ga0.8As, an n-type-impurity-doped electron supply layer 105 made of InGaP, and an n-type-impurity-doped cap layer 107 of GaAs having an opening formed to expose the electron supply layer 105 are successively stacked on a compound semiconductor substrate 101 made of GaAs. On the electron supply layer 105 exposed at the opening of the cap layer 107, a gate electrode 108 is formed by a Schottky junction. Source and drain electrodes 109 and 110 are formed on the cap layer 107.
InGaP is a material with a lower interface state density than AlGaAs. Thus, the interface state level of the electron supply layer 105 exposed at the opening of the cap layer 107 can be reduced. This can enhance the maximum current density in the application of an RF signal.
By the way, when a semiconductor layer of InGaP (InGaP layer) in a known MODFET is crystal-grown through a commonly used compound semiconductor fabrication method, a natural superlattice is formed in which Ga atoms and In atoms are alternately placed in the same plane in Group-III atomic layers.
In recent years, a method for forming an InGaP layer by breaking a natural superlattice structure to disorder the arrangement of Ga atoms and In atoms in the Group III atomic layers has been known, for example, as described in Japanese Laid-Open Patent Application Publication No. 11-243058. Field-effect transistors have been developed in which such InGaP with a broken natural superlattice is used to reduce the interface resistance between an InGaP layer and the adjacent semiconductor layers.
However, it has experimentally been known that the known MODFET above has a lower reverse breakdown voltage of the gate electrode 108 to the drain electrode 110 than that using AlGaAs for the electron supply layer 105. The reason for this is that in the known MODFET, the gate electrode 108 is formed by a Schottky junction with the electron supply layer 105 made of InGaP.
In this way, when in the known MODFET InGaP is used to enhance the maximum current density in the application of the RF signal, the material constraints of InGaP reduce the gate breakdown voltage. Therefore, it is difficult to improve both the maximum current density and the gate breakdown voltage.